Dielectric copying sheet

ABSTRACT

IMPROVED LIGHTWEIGHT DIELECTRIC COPOLYING SHEET INCLUDED A BASED OF CRAZED POLYSTYRENE HAVING A CONDUCTIVE RESIN COATING.

United States Patent Office 3,746,574 Patented July 17, 1973 US. Cl. 117-201 7 Claims ABSTRACT OF THE DISCLOSURE Improved lightweight dielectric copying sheet including a base of crazed polystyrene having a conductive resin coating.

DESCRIPTION OF THE INVENTION The invention relates generally to dielectric copying material. More particularly, the invention relates to an improved dielectric copying material having a synthetic case of crazed, oriented polystyrene.

Dielectric copying papers conventionally are made by coating a cellulosic base sheet with a nonconductive coating on the imaging side and a conductive coating on the opposite side. The base sheet generally has a moderate value resistivity of about ohms at 50% relative humidity, for example. The nonconductive or dielectric coating usually is about 0.5 to 1.0 mil in thickness providing a surface resistivity of about 10 ohms/sq. The coating on the other side is usually about 0.05 to 0.5 mil thick with a resulting surface resistivity of about 10 ohms/ sq. Such copying papers are well known and will not be further described here.

The imaging mechanism is also well known but will be described briefly. The dielectric sheet is placed on a metal plate which is connected to the negative pole of a DC. source with the conductive side of the sheet contacting the metal. A dielectric image can be produced on the dielectric (nonconductive) side by various methods. For example, writing on the sheet with a stylus connectedto the positive side of a DC. source is one convenient way. The image is then developed by immersing the sheet in a negatively charged bath of a toner dispersed in a solvent. Toners are readily available for such use and generally include finely divided, well dispersed, charged, dyed particles in a carrier such as kerosene. The particles may be, for example, low molecular weight polystyrene. Important for the quality of the image is that the thickness of the layers be uniform so as to provide uniform conductivity. This uniformity has proved extremely diflicult to achieve wtih conventional paper base materials.

Accordingly, it is an object of the present invention to provide an improved light weight dielectric copying sheet imaging properties. Further, it is an object of the invention to provide such a sheet requiring a single coating.

Other objects and advantages of the invention will become apparent to those skilled in this art by reference to the detailed disclosure below.

In accordance with the invention the improved dielectric copying sheet includes a base material of oriented polystyrene. The particular properties of the polymer film are not critical; however, it preferably has the following characteristics: thickness from about 0.5 to 2.5 mils, degree of orientation 1.2x by 1.0x, to 6x by 6x. Thus, the base may be uniaxially oriented in either direction or biaxially oriented up to about six times in either or both directions. Alternatively the base may be a coextrusion or laminated product having oriented polystyrene on the outer or major surfaces. Polystyrene is readily available, and, for example, includes Styron 685 manufactured by The Dow Chemical Company.

Further in accordance with the invention the oriented polystyrene base is crazed on both surfaces so as to make it opaque and increasingly receptive to coatings. The crazing process for polystyrene is known and generally involves contact with a solvent mixture composition to produce small voids in the surface of 0.1 to 2 microns in diameter, for example. Further details as to the crazing of polystyrene may be obtained by reference to US. Pat. 2,578,770.

The crazed, oriented polystyrene is coated on one side with a conductive resin. These resins are also known and include, polyvinyl-benzyl trimethyl ammonium chloride; polysodium styrene sulfonate, and others listed, for example, in an article entitled Electrocond-uctive Polymers" by Delinski and Dean published in Chem. Tech, May 1971, p. 304+. It is preferred that the solvent for the conductive resin wet the substrate well without dissolving it so as to promote the deposition of the coating well into the small voids caused by crazing. Thus the preferred solvent compositions include low molecular weight alcohols and low molecular weight hydrocarbons.

Example The following example is given for the purpose of illustrating the invention which is not limited thereto as various modifications and equivalents will be apparent to those skilled in this art.

A 1.5 mil biaxially oriented polystyrene (Trycite 5000 by The Dow Chemical Company) film was crazed on both sides by immersing it for about 0.2 second in a mixture of 4 parts by volume acetone and 1 part by volume isopropanol. Samples of this crazed sheet material were subsequently coated on one side with a series of methanolwater solutions of polyvinyl-benzyl trimethyl ammonium chloride with the results shown in Table I. Surface resistivity tests were performed with Kiethyl resistivity apparatus at 5 0% relative humidity.

TABLE I Coating Methanol/ Percent by solution, g water, weight resin Surface resin per volume deposit on resisitivity,

Sample cc ratio sheet ohms/sq.

In all cases samples were imaged with a stylus charged with 700 volts and developed a high quality sharp image when treated according to conventional procedures with a toner bath.

From the foregoing it can be seen that the sheet of the invention provides high quality copying sheet with relatively low coating weights. Further, the sheet of the invention exhibits relatively low surface resistivity so as to provide for enhanced imaging and is lightweight. While the quality of the image depends, in part, upon the relative resistivities of the opposing surfaces, the present invention provides resistivities of less than 10 ohms/sq. on the coated side thus allowing lower applied voltages for imaging than are required for conventional dielectric copying sheets.

We claim:

1. Dielectric copying sheet consisting essentially of,

a relatively nonconductive substrate having at least one major surface formed of crazed, oriented polystyrene, and

a conductive coating having a surface resistivity of less than 10 ohms/ sq. on said polystyrene surface.

2. The dielectric copying sheet of claim 1 wherein the substrate is a laminate having oriented polystyrene on both major surfaces.

3. The dielectric copying sheet of claim 1 wherein both of the major surfaces of said substrate are formed of crazed, oriented polystyrene.

4. The dielectric copying sheet of claim 3 wherein said crazed surfaces contain voids having a diameter of from about 0.1 to 2 microns and said coating is deposited well within the voids on said one surface.

5. The dielectric copying sheet of claim 3 wherein said substrate consists essentially of a polystyrene film having a thickness of from 0.5 to 2.5 mils.

6. The dielectric copying sheet of claim 5 wherein said polystyrene is oriented from 1.2x to 6x in one direction and from 1x to 6x in the other direction and has a thickness of about 1.5 mils.

7. The dielectric copying sheet of claim 6 wherein the coating contains polyvinyl-benzyl trimethyl ammonium chloride as a conductive resin.

References Cited UNITED STATES PATENTS ALFRED L. LEAVITI, Primary Examiner M. F. ESPOSITO, Assistant Examiner U.S. Cl. X.R.

2 25 3 UNITED STATES PATENT OFFICE v CERTIFICATE OF CORRECTION Patent No. 3,746,514 Dated July 17,1973

Inventor(s) Eckhard C. A. Schwarz and Philip R. Bartels It is certified that error appears in the above-identified patent s shown below:

and that said Letters Patent are hereby corrected a "value" should read volume Column 1, line 50, "wtih" should read with Column 1, line 52, after "sheet", insert having highly uniform base and coating thicknesses and excellent Column 1, line 25,

Signed andfsealed this- 2nd day of ApI'il- 197R (SEAL) Attest: c. MARSHALL DANN Commissioner of Patents EDWARD M.FLETGHER,JR. Attesting Officer 

